1. Field of the Invention
The present invention relates to the field of high intensity focused ultrasound (HIFU) monitored by magnetic resonance imaging (MRI) and, particularly, to a method for reducing errors in the measurement of temperature of MRI guided HIFU equipment.
2. Description of the Prior Art
Magnetic resonance (MR) thermometry based on proton resonance frequency (PRF) shift can be used to monitor temperature changes in an area heated by HIFU in MRI-guided HIFU equipment, based on the phenomenon of the resonance frequency of the protons in water being offset (shifted) dependent on the temperature change. MR thermometry based on PRF-shift requires that a base image (MR phase image) before heating, also referred to as a reference image, be generated, with the reference image providing information on a reference phase. By subtraction from the phase image (also referred to as a heated image) acquired during heating or after heating, the exact value of the elevated temperature in the heated area can be determined.
During a practical heating process, however, after the reference image is acquired changes may occur in the position of the ultrasonic transducer (i.e. the treatment head), and the susceptibility change resulting from the movements of the ultrasonic transducer causes changes in the static magnetic field of the focal region of the ultrasonic transducer, so that the subtraction of the heated image and the reference image produces an additional phase difference, thus causing errors in the temperature measurement.
Currently, there are mainly two common solutions for reducing temperature errors. One of the solutions can be referred to as a single reference image method wherein after the reference image has been acquired, the movement range of the ultrasonic transducer is restricted, so as to restrict the temperature errors within an acceptable range. However, since the spatial range used by a reference image is very small, while the ultrasonic transducer moves within a relatively large spatial range in the HIFU treatment process, it is necessary to acquire reference images frequently for various positions in order to measure the temperature of each focal position of the ultrasonic transducer, and this increases the complexity of the temperature measurement and the overall treatment time.
The other solution for reducing temperature errors can be referred to as a self-reference method, i.e. not acquiring any reference images, but instead utilizing the heated images themselves to obtain the reference phase by a polynomial fitting and extrapolation of the phase from the non-heated region. The temperature change monitored using this method is limited to the vicinity of the focus of HIFU, and it is very difficult in practical applications to monitor the temperature changes outside the focus point. Furthermore, the accuracy of the polynomial fitting and extrapolation and the complexity of the phase image are dependent on the size of the heated area, and it is relatively difficult to obtain stable, consistent and accurate results in general.